An electric load such as a DC motor is driven by pulse width modulation (PWM) by a switching element such as a power MOSFET, which is connected in series to the load. In this case, noise is generated by the switching operation of the switching element. The PWM control is performed at a carrier frequency, which is generally set in a several kHz range. In case of driving a motor mounted in a vehicle, for example, it is required, to take a measure so that radio communication devices such as a car radio receiver are not adversely influenced by the noise. Patent document (JP 2008-5682A) proposes, for countering this problem, to vary the carrier frequency in a predetermined range so that a peak of the noise will not concentrate on one frequency.
The technology disclosed in JP 2008-5682A reduces a noise, which affects a radio receiver of an AM frequency band. However, it does not reduce noise, which affects a radio receiver of a FM frequency band. The noise generated in a MHz frequency band is dependent on a change rate or gradient, that is, a slew rate of a waveform of a driving voltage, which turns on and off the switching element. It is possible to reduce the frequency of generated noise by decreasing the gradient of the waveform. This however prolongs a turn-on and turn-off period of the switching element and increases switching loss.